In an image forming apparatus for thermally fixing a toner image formed by electrophotography on a printing paper sheet, the fixing temperature of the fixing device is decided in accordance with the color material amount per unit area to be placed on the printing paper sheet. Normally, the maximum value of the color material amount per unit area is predetermined, and the temperature is adjusted to obtain a fixing temperature capable of properly fixing an image having the maximum value.
In a full-color copying machine, since image formation is done by superimposing a plurality of color materials such as CMYK color materials (cyan, magenta, yellow, and black, respectively), the color material amount (to be referred to as a toner amount hereinafter) to be placed on a printing paper sheet tends to be large. For this reason, the required heat capacity of a fixing roller increases. If the temperature of the fixing device is lower than a predetermined set temperature after powering on the machine, after being in the sleep mode, or the like, the warm-up time up to the predetermined set temperature is long. As a result, a waiting time to the start of printing occurs. In addition, an image may be output in a toner amount much smaller than the assumed maximum amount. For example, in a mode using only the K color material, heating is excessive, and the power is wasted.
To suppress the above-described power consumption, in Japanese Patent Laid-Open No. 2004-245895, when draft printing is selected, image data undergoes thinning. A temperature lower than that in any mode other than the draft printing is determined to be the fixing temperature in accordance with the image data thinning ratio.
An image forming apparatus has an image adjustment function represented by a density adjustment function. For example, if a printed product seems to be light-colored, the set value of density is changed to be larger by the image adjustment function, and reprinting is performed.
In the above-described arrangement, the toner consumption increases when the set value is changed to a high density. The fixing temperature changes to a high temperature when the toner consumption increases. As the result of the change in the fixing temperature, the image appearance, including the tonality and gloss characteristic, changes before and after the image adjustment. The gloss characteristic changes due to a change in the printed surface properties caused by toner fusibility conversion. If such a change in the tonality and gloss characteristic occurs during an adjustment value search while getting feedback from the printed product, the optimum adjustment value is difficult to acquire.